Composite materials based on epoxy resins are used in a variety of applications and continue to have considerable importance because of their versatility. A specific example of such an application is in the production of electrical laminates used in printed circuit boards (printed wiring boards, PWB). These electrical laminates are typically prepared, for example, from a fibrous reinforcement and an epoxy-containing matrix resin in a multi-step process.
For example, U.S. Pat. No. 6,403,220 and U.S. Pat. No. 6,645,631 describe a general process suitable for the preparation of electrical laminates in which (1) an epoxy-containing formulation is applied to or impregnated into a substrate, e.g., a woven or nonwoven fiber mat containing, for instance, glass fibers or paper, which substrate is then (2) heated at a temperature sufficient to draw off solvent in the epoxy formulation and optionally to partially cure the epoxy formulation. This heating step is known as “B-staging” and the product is known as a “prepreg”, which is, as a result of “B-staging”, more easily handled in subsequent manufacturing steps wherein (3) one or more sheets of prepreg are stacked or laid up in alternating layers with one or more sheets of a conductive material, such as copper foil, if an electrical laminate is desired, and pressed at elevated temperature and pressure for a time sufficient to cure the resin and form a laminate.
Common temperatures for the “B-staging” step are from about 90° C. to about 210° C. for a time ranging from about 1 minute to about 15 minutes, but other temperatures and times can be used.
A typical requirement for these laminates, as for many other applications, is flame resistance, and a fire retardancy level of V-0 in the standard “Underwriters Laboratory” test method UL 94 is typically desired. To achieve this level it is typically necessary to incorporate flame retardant compounds into the epoxy resin, which can conveniently occur when preparing the epoxy formulation applied or impregnated in step (1) above. Halogen containing compounds, such as tetrabromobisphenol A, are widely used. The laminates obtained using this material deliver properties and performance characteristics that are well-understood.
There is an increasing interest in non-halogen containing flame-retardants. However, these replacement materials must still be able to meet not only flame retardancy requirements, but also the physical requirements needed for individual applications, such as the mechanical properties, toughness, and solvent and moisture resistance desired in epoxy compositions that can be obtained using halogenated materials.
Phosphorus based flame retardants have been used as alternatives to halogenated flame retardants. Many such compounds are known, a number of which are commercially available. In order to avoid problems associated with loss of flame retardant, e.g., through exudation etc, reactive derivatives of phosphorus flame retardants which can react into the polymer resin are known and used in epoxy formulations. For example, phosphorus based flame retardants bearing groups such as hydroxy, amino, epoxy, vinyl, carboxylate etc., have been used in the preparation of flame resistant epoxy resins.
Reactive phosphorus containing flame retardants have been incorporated into epoxy resins by straightforward reactions to produce a phosphorus containing epoxy resin which are then cured using cross linking agents and standard methods. Alternatively, the phosphorus based flame retardant can be blended into a curable epoxy formulation as a reactive component, such as a crosslinker, and incorporated into the resin during cure.
Alkyl and aryl substituted phosphonic acid esters are compatible with epoxy resins but are also known plasticizers. Laminates formed therefrom tend to exhibit low glass transition temperatures, which are often unsatisfactory for use in electrical laminates. Further, the use of phosphonic acid esters in amounts sufficient to provide the necessary flame retardancy increases the tendency of the resulting cured epoxy resin to absorb moisture. The moisture absorbency of cured laminate board is significant because laminates containing high levels of moisture tend to blister and fail when subjected to the soldering operations typically employed in the manufacture of printed wiring boards.
Various other phosphorus based flame retardant materials are described in the literature, for example, EP 0 754 728 discloses a cyclic phosphonate, EP 1 116 774 uses a hydrogen phosphinate, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, in conjunction with triphenylphosphine oxide, but their use in the manufacture of printed wiring boards tends create the difficulties above or have specific requirements that greatly add to the expense of manufacture.
Phosphine oxides have also been used as flame retardants in resins such as epoxy resins. Phosphine oxides containing groups capable of reacting into polymer resins such as epoxide, polyester, polyamide, polycarbonate etc, are also known, as is their use as flame retardants and cross linkers.
U.S. Pat. No. 4,973,631 discloses a triphenyl phosphine oxide curing agent of the formula
where each Ar is a substituted or unsubstituted phenyl ring and X is an epoxy-reactive substituent having active hydrogen, for example, amine, hydroxy, carboxy, anhydride, and thiol moieties.
JP 2000186186 A discloses compositions comprising epoxy resins, cross linking agents and organophosphorus compounds having one or more P—C covalent bonds and 2 or more hydroxy groups, for example, bis(p-hydroxyphenyl)phenylphosphine oxide and tris-(p-hydroxyphenyl)phosphine oxide.
U.S. Pat. No. 6,403,220 discloses mixtures comprising isomeric mixtures of unsubsitituted and alkyl substituted tris-(ortho-hydroxyphenyl)phosphine oxides and their use as cross linkers in epoxy resin compositions. JP 05057991 discloses compositions comprising epoxy resins and meta-hydroxyphenyl phosphine oxides.
U.S. Pat. No. 6,733,698 discloses a mixture of hydroxyarylphosphine oxides comprising (a) a mono(hydroxyaryl)phosphine oxide, (b) a bis(hydroxyaryl)phosphine oxide, (c) a tris(hydroxyaryl)phosphine oxide, and, optionally (d) a tri-aryl, alkyl or aralkyl-substituted phosphine oxide useful as a crosslinker/flame retardant in, e.g., epoxy resins.
The above compounds can also be derivatized prior to incorporation into epoxy resins. For example, reaction between the phenoxy group and epichlorohydrin provides a useful epoxy functionalized material.
Mixtures of hydroxyarylphosphine oxides, such as those found in U.S. Pat. No. 6,733,698 and co-pending U.S. Pat Appl No. 12/807,642 and Ser. No. 12/857,994, are effective as both flame retardants and cross linkers in epoxy resins such as those used in electrical laminates. For example, they are readily formulated into curable epoxy compositions used in the formation of laminate prepregs, typically with phenolic-containing hardeners and amine catalysts. The cure rate of these formulations can be extremely fast and in some instances is too fast to allow for robust processing conditions. Greater control over the cure time would allow one to slow the rate of curing providing greater production flexibility and allow for a more fully optimized manufacturing process. For example, increasing the time during which a prepreg remains partially cured and easily handled, i.e., the ‘gel time’, would greatly improve process control over laminate production.
Combinations of amines with Lewis acids, especially boron compounds such as BF3, boric acid and derivatives thereof, have long been used in the curing of epoxy resins. U.S. Pat. No. 6,645,631 describes Lewis acids, preferably boron Lewis acids, as inhibitors when used with amine catalysts, possibly suggesting that the role of the Lewis acid is to slow the cure rate by inhibiting the activity of the catalyst.
It is found that aluminum salts of certain organo-phosphorus acids, not previously disclosed as cure inhibitors, are very effective cure inhibitors for epoxy formulations.